The application generally relates to a vapor compression system. More specifically, the application relates to a system and method to control the intermediate pressure in an economized vapor compression system.
A vapor compression system can include refrigerant gas compressed by a compressor and passed to a condenser where it exchanges heat with another fluid, for example, air or water, and is condensed to a liquid. From the condenser, the liquid refrigerant can pass through an expansion device and then to an evaporator, where the refrigerant exchanges heat with another fluid, for example, air or water, and is evaporated to a gas. The refrigerant gas can return to the compressor from the evaporator and the cycle is repeated.
Economizer circuits can be utilized in vapor compression systems to provide increased cooling capacity, efficiency, and performance when compared to systems without economizer circuits. An economizer circuit utilizing one or more additional expansion devices can be incorporated downstream of the condenser. For a system utilizing one additional expansion device, a first expansion device can expand the refrigerant from condenser pressure to an intermediate pressure between condenser pressure and evaporator pressure, resulting in the flashing of some of the refrigerant to a vapor. The flashed vapor refrigerant can be provided back to the compressor. The remaining liquid refrigerant at the intermediate pressure from the first expansion device is now at a lower enthalpy. A second expansion device can expand the lower enthalpy liquid refrigerant from the intermediate pressure to evaporator pressure. The refrigerant can then enter the evaporator with lower enthalpy, thereby increasing the cooling capacity of vapor compression systems with economizing circuits versus non-economized systems in which the refrigerant is expanded directly from the condenser.
An economized vapor compression system can also include a flash tank and an additional expansion device. In flash tank economizer circuits, the first expansion device is provided upstream of the flash tank. Liquid refrigerant flows through the first expansion device and into the flash tank. Upon entering the flash tank, the liquid refrigerant experiences a substantial pressure drop, and at least a portion of the refrigerant rapidly expands or “flashes” and is converted from a liquid phase to a vapor phase at an intermediate pressure. Any remaining liquid refrigerant gathers at the bottom of the tank for return to the main refrigerant line upstream of the second expansion device. Vapor refrigerant is returned from the flash tank to the compressor, either at the suction inlet to the compressor or at an intermediate stage of compression. Since the refrigerant gas returned to the compressor from the flash tank is at an intermediate pressure, the refrigerant vapor requires less compression, thereby increasing overall system efficiency.
Introducing gas refrigerant from a flash tank economizer to a compressor suction inlet or to an intermediate stage of compression in a multi-stage compressor can be problematic. A first stage compressor may handle the flow from the evaporator while a higher stage compressor handles the flow from the first stage compressor discharge as well as the flow from the economizer. In conventional systems, the economizer operating conditions can be dictated by the overall system conditions and operating point, thereby prohibiting independent control of the economizer operating pressure and flow rate. Without such independent control, the economizer and second stage compressor must be designed for specific operating conditions. Further, introducing gaseous refrigerant from the economizer circuit in systems having only a single-stage compressor can be problematic because there is no mechanical device or method to operate the compressor at a pressure level between the evaporator and condenser.